POSTERIOR TILTING OF THE TIBIAL COMPONENT DECREASES FEMORAL ROLLBACK IN POSTERIOR-SUBSTITUTING KNEE REPLACEMENT - A COMPUTER-SIMULATION STUDY

Posterior tilting of the tibial component is thought to increase the r ange of motion in posterior cruciate-retaining total knee replacement, but its effect on implant motion in posterior cruciate-substituting t otal knee replacement is unknown. This issue has become of interest re cently because manufacturers have introduced instrumentation that prod uces a posteriorly tilted tibial cut for both implant types. The purpo se of this study was to investigate how motion of posterior cruciate-s ubstituting total knee replacement is affected when the tibial compone nt is installed with posterior tilt. Sagittal plane implant motions we re predicted from prosthesis geometry with use of a computer simulatio n in which the femoral condyles were assumed to sit in the bottoms of the tibial condylar wells when the knee was in extension. Rollback of the femoral component was produced by a cam-spine mechanism at higher angles of flexion. The simulations revealed that even small degrees of posterior tilt reduced rollback by limiting the interaction between t he cam and spine. Tilting the component posteriorly by 5 degrees cause d the cam to contact the spine at a knee flexion angle that was 18 deg rees higher than with the untilted component. The results suggest that posterior tilting of the tibial component in posterior cruciate-subst ituting knee replacement may not produce the same beneficial effects t hat have been reported for the tilting of tibial components in posteri or cruciate-retaining knee replacement.